DATA

Bits
Everything you see on your computer screen is a digital representation. All computer data is expressed in bits, which are either 0's or 1's. "We might call these values 0V and 3.5V, or on or off, or true or false, or yin or yang, but we conventionally denote them as 0 and 1."

Bits, Bytes and Displays
Bits are grouped into larger units called bytes. There are 8 bits to one byte. One byte may have the binary number 01100001 (8 bits) which is equal to the value 97. This number can be translated into a color, a letter, a brightness or whatever the software and hardware and their interface determines it to be. In the ASCII character set it assigns the bit pattern 01100001 (value 97), the lower case letter "a". When you press the key "A" on your keyboard, it sends an electrical pattern (0's and 1's) or the value 97 to your computer and software to display "a".

In terms of color depth, or the complexity of color in an image, the number of bits determines the color displayed. A single bit is two colors; 4 bits produces 16 colors; 8 bit color depth means that each pixel could one of 256 colors. Each 256 colors has a number or value associated with it; for instance, 0 = Black and 255 = White. You have probably noticed this if you use Photoshop and color sliders. 64 color bit depth means each pixel could be one of a million plus colors, giving you a wide range of hue values. However, the more bits, the larger the file size--for 64 bit color (millions of colors), each pixel requires 64 bits, rather than 8 bits for 8 bit color. The more color variation, the larger the file size. Before broadband was developed or commonly used, many web designers used 2 or 4 bit color which produced interesting graphics. This is one of the ways that limitations of the web can be used creatively. For a good description of various bit depths and the difference between using gif or jpeg, follow this link.

Data Rates
In streaming media, we talk about data rates, the amount of data which moves through a connection, in terms of bits or kilobits or kbits or kb. This is not to be confused with KB or Kilobytes or KBytes or KB; this expression is more often used with multimedia files in which data transfer rates can be much higher--in the megabytes rather than bits. For streaming purposes, we almost always speak in "bits".

However, here are some numbers to remember:

8 bits = 1 byte
1000 bits = 1 kilobit (kbit) (a 56k Modem is 56kbits/sec; or 56, 000 bits a sec--theoretically)
1024 bytes = 1 Kilobyte (KByte or 1K)
1000 KB = 1 Megabyte (MB)
1000 MB = 1 Gigabyte (1 GB)

Digital Video's (DV) transfer rate is 3.6 MBytes per second. (30,000 kbits a second)
1 minute of uncompressed DV = 200 MB
5 minutes of uncompressed DV = 1 GB

Data Rates and Network Connections
If you look in your reader, at Cleaner 6 Manual Pages, a. Data Rates, you will see common bit rates for different network connections. These are the network bandwidths, or how much data can pass through the connnection from the server to your or anyone's computer. Think of this as pipes though which water flows. The larger the pipe the more water (more data) and the least resistence (faster, quieter). If you look at the 56.6k modem connection, it is about 32 kbits per second (kbps). (or 4 KBytes. Do the math--8 bits to the byte, so divide 8 into 32 to get 4KB. However, it is always better to use kbits when discussing network connections.)

Below are some connection speeds and what you might expect. Actual throughput is of course effected by complexity of video image, size of image, type of encoding, length of video and number of people connected to stream and so on. Different manuals will give you different data rates. Many providers oversell the capacity for connectivity to their server, sometimes up to 10 to 1. A 300 kbps DSL connection may have the actual throughput of a 28.8 modem in real life.

Want to know what kind of real connectivity you have at home? Try downloading a large uncompressed file, measured usually in Kbytes (such as one of the players we are using). Multiply this by 8 (8 bits = 1 byte), then divide this by number of seconds it took to download. This would be your current throughput. Try this at different times of the day.

Compression Strategies
In general we can reduce file size in three ways:
1. Reduce the data-rate by increasing compression
2. Reduce the physical size or pixle dimensions of the video
3. Reduce the number of frames per second
As for #1, compression can happen either spatially or temporally, or both. Different codecs apply different compression schemes, some relying more heavily on spatial compression (compression within the frame) and some on temporal compression (compression across or between frames). The trick is to experiment with all three of these factors, until you find a combination which suits your need for your particular video. Spatial compression is often expressed in export options (in FCP) as "quality", where as temporal is expressed through "keyframes".

For more information on compression basics, visit this website.